The Digestive System 137 



The relationship of the stomach to the mesenteries is rather com- 

 pHcated due to the manner in which the stomach has become modified 

 during development. Extending from the stomach is a large sheet 

 of mesentery known as the greater omentum, which in part supports the 

 stomach. Most of it extends posteriorly to form a bag which hangs 

 down in front of the intestine in some mammals. In man, fat is very 

 often stored here, at times forming an impressive frontage on older men. 



Glands. — It has already been pointed out that numerous glands 

 are present in the walls of the digestive tract. In addition, there are 

 three other important glands closely associated with the functioning 

 of the tract. The first of these are the salivary glands which open into 

 the mouth cavity. 



The largest gland of the body is the liver. It is located just below 

 the diaphragm and in man is composed of four lobes. It secretes the 

 important digestive juice, bile, which empties into the duodenum through 

 the common bile duct. In those animals (including man) which have 

 a gall bladder, the bile may be stored temporarily until needed. In 

 addition to its digestive function, the liver also performs other essen- 

 tial tasks such as storage of sugars, removal of waste substances from 

 the blood, and many others. 



Another essential digestive gland of the body is the pancreas, a 

 pinkish, lobular organ located just below the stomach in the mesen- 

 teries. Its secretion, pancreatic juice, is emptied into the duodenum. 

 In some mammals, the pancreatic duct has its own opening into the 

 duodenum; in others it joins the common bile duct. In addition to 

 secreting pancreatic juice, the pancreas secretes a hormone, insulin, 

 which is important in sugar metabolism. The pancreas thus has two 

 entirely unrelated functions. 



THE PHYSIOLOGY OF DIGESTION 



The function of the digestive tract is to change ingested food into 

 simpler forms which can be utilized by the body. This change is made 

 possible by two interrelated processes, one mechanical, the other chemi- 

 cal. The chemical processes, brought about by the enzymes, split the 

 food molecules into simpler soluble ones; the mechanical processes 

 move the food along, break it into smaller portions, and mix it with 

 the digestive juices. 



